1. Field of the Invention
This invention relates generally to optical methods and apparatus for determining the size and quantity of particles and bubbles in liquid mediums. More specifically it relates to the monitoring and/or analysis, either real-time or off-line, to detect and to distinguish particles and bubbles in process liquids used in manufacturing applications where the purity of a liquid medium is important, such as semiconductor or medical applications.
2. Brief Description of the Prior Art
Particles are generated in the manufacture of semiconductor devices, which contaminate liquids used during wafer fabrication, such as chemical etches, photo resists deionized water and solvents, used during wafer processing, generate particles. The number of defects and thus process yield is known to be directly proportional to the presence and number of particles in the liquid medium used for processing. Modern wafer processing methods require minimizing the amount of particulate matter in all liquids in contact with the wafers. Thus process mediums are monitored for particle contamination and various filtering methods are utilized to remove particles.
Since bubbles generate signals which are indistinguishable from particles in conventional methods for monitoring semiconductor liquids during processing, bubbles are measured and counted as particles.
Particles larger than 1 micron can be successfully filtered, however reliable removal of particles smaller than 1 micron becomes progressively more difficult. As the art of wafer processing advances, purity standards for the processing medium become more stringent and the detection of particles down to 0.1 micron or less is desirable.
No satisfactory in-line method for physically separating bubbles from sub-micron particles for wafer processing has been found.
Because of the difficulty in discriminating between particles and bubbles, prior art devices count both particles and bubbles which are not distinguishable from particles, thus the count is a pseudo particle count rather than a true particle count. Because of this pseudo particle count the process medium is prematurely determined to be exhausted and rejected.
In many applications used process liquid is environmentally harmful and often the appropriate methods for disposal require expensive special handling. The premature rejection of process liquid also increases the cost of processing.
When the particle count exceeds the allowable level, the number of defects increases and the process yield declines.
Particle monitors exist which measure off-line samples of the process medium or which are positioned in the process bath recirculating lines, however due to particle dynamics such monitors, in reality, measure filter efficiency and not on-line bath conditions. Furthermore, such monitors do not distinguish between particles and bubbles.
A requirement of any sampling device which is on-line and in contact with the process medium is that the sampling system not generate or introduce additional particles. The materials which contact the process medium must be non-reactive within the medium.
The principal object of this invention is a method and device for reliably detecting the presence and quantity of sub-micron particles in the presence of bubbles.
Another object of this invention is the detection of particles of down to 0.1 micron or less, within liquid mediums.
It is another object of this invention to more efficiently detect the point at which the particle count has exhausted the effectiveness of the medium, thus eliminating premature rejection and reducing the disposal problems.
It is yet another object of this invention to accurately detect the point at which the quantity of particles causes an unacceptable yield and to signal the process controller to change the medium.
It is still another object of this invention to provide a real-time on-line measurement of particles.
A further object of this invention is to monitor bacteria count in the presence of particles and bubbles.
Yet another object of this invention is to provide a sample handling device and method which is unobtrusive, which does not generate additional particles, which is protected from toxic substances within the bath, and which assures a representative sample.
Hatton and Plawsky in U.S. Pat. No. 4,662,749 Fiber Optic Probe and System for Particle Size and Velocity Measurement, disclose and discuss prior art methods and apparatus for measuring particles. Tatsuno in U.S. Pat. No. 4,595,291 Particle Diameter Measuring Device, discloses the use of laser beams and optical fibers for directing a light source and monitoring light scattered by particles for particle measurement. Philip H. Paul and George Kychakoff in "A Miniature Fiber-Optic Probe for Optical Particle Sizing" Journal of Lightwave Technology Vol. LT-5, No. 7, July, 1987 discuss the use of laser beams and detection of forward and right angle scattered light for measuring particle size.